The use of thread-forming screws for securely coupling bodies is well known. This type of screw is inserted in an unthreaded bore or aperture and forms threads in the bore or aperture when a torque is applied. The screw structure is highly vibration-resistant for maintaining the coupling between the bodies even when subjected to considerable stress. Some self-threading screws include threads with multiple lobes around the circumference of the screw shank. The lobes, or undulations, further resist loosening of the screw.
Considerable torque is required in forming the threads on the screw's shank. The greater the required torque, the greater the required strength of the screw. The application of increased torque in the threading process as well as providing a screw comprised of materials of greater strength increase the cost of the end product. The goal is, therefore, to form the threads on the screw with a minimum torque. Where the screw is tapered, additional difficulty is encountered in providing the tapered portion with crested, or fully formed, threads. Threads which are not crested not only reduce the stripping torque required to remove the screw, but also render it more difficult to insert the screw. The size, shape and spacing of the threads on the tapered end of a thread-forming screw are critical to the ease with which it is inserted in an unthreaded bore or aperture because it is this portion of the screw which initiates the threading process.
The present invention affords the aforementioned advantages in a thread-forming screw which is formed from a cylindrical blank passed through a tapered thread rolling die so as to form fully crested tapered threads on its end with improved stress relief for facilitating the tapering and threading operation.